This invention concerns a procedure for preparing amorphous silica from calcium silicate.
From the literature, many methods are known for obtaining amorphous silica from various materials. The starting materials used are generally silicates of alkaline metals, preferably sodium, as described for example in W096/30301.
A particularly interesting starting material is calcium silicate.
Natural or artificial calcium silicates are low-cost materials which by means of the reaction of CO2 in water, allow solid mixtures of SiO2 and CaCO3 to be obtained, products with a high added value.
However, the use of calcium silicates instead of silicates of alkaline metals, or particularly instead of sodium silicate, presents considerable problems due to the poor solubility of calcium silicate in water, in comparison with sodium silicate.
An example of a procedure for obtaining silica and compounds of silica and calcium carbonate from calcium silicates is described in patent CA 1,122,779. That patent describes a procedure for obtaining silica, in which the calcium silicate crystals are placed in contact with CO2 in the presence of water and converted into silica, having the same configuration as crystals of silicate, and into particles of calcium carbonate attached to the particles of amorphous silica. The SiO2-CaCO3 mixture is not separable and so, to recover the silica, a treatment with mineral acids is carried out. The acid decomposes the calcium carbonate into CO2 and calcium salts. The acid may be HCl, for example, thus obtaining CO2 and calcium chloride. It is then washed with water to eliminate the calcium salt, thus obtaining an aqueous suspension containing silica. However, with a procedure of this type the quantity of silica obtained is extremely low in comparison with the volumes of liquid involved. There is therefore a low production rate per unit volume, which considerably penalises the process described even though it allows particularly pure silica to be obtained.
Moreover this procedure requires the use of an inorganic acid and it is not possible to recover the CaCO3.
The aim of this invention is therefore to develop a procedure for obtaining amorphous silica from calcium silicate which presents high yield and a high production rate, along with high purity of the product.
A further aim of this invention is to obtain a process with a low degree of environmental pollution, since instead of mineral acids it uses CO2 alone as the acid agent. A further aim of this invention is to recover precipitate calcium carbonate in mild temperature and pressure conditions.
The aim of this invention is therefore a procedure for preparing amorphous silica comprising the following phases:
a) reaction of a calcium silicate with CO2 in an aqueous environment with the formation of a suspension 1 of agglomerated particles of SiO2 and CaCO3;
b) treatment of the suspension 1 with a compound of aluminium, boron or zinc or mixtures of the same in a neutral or basic environment, and formation of a solid phase 2 in a solution 3 containing particles of SiO2 with nanometric dimensions;
c) separation of the solid phase 2 from the solution 3; and
d) treatment of the solution 3 according to one of the following methods;
e) precipitation or drying;
f) gelation.
In the description and the claims of this invention the expression xe2x80x9camorphous silicaxe2x80x9d means non crystalline silica which may be obtained in the form of a gel or in the form of a precipitate.
The reaction of phase a) of the process is carried out in an autoclave at a pressure between 0.3 MPa and 3 MPa and at a temperature between 10xc2x0 C. and 100xc2x0 C. More particularly the phase a) is carried out at a pressure between 1.0 and 2.5 MPa and at a temperature between 15 and 40xc2x0 C. Preferably the pressure is equal to 2 MPa and the temperature is equal to 20xc2x0 C. or 30xc2x0 C.
The pH is a function of the CO2 pressure and, around a pressure of 2 MPa, it stabilises around the value 5.2. The water/calcium silicate weight ratio may have any value, preferably between 16 and 6.4.
In these conditions of temperature, reagent concentration, pressure and pH, the reaction proceeds with the dissolution of the calcium silicate and concludes with the formation of an aqueous suspension 1 of agglomerated particles of SiO2 and CaCO3.
The simultaneous precipitation of SiO2 and CaCO3 could be avoided by carrying out the reaction with low loads of calcium silicate (water/calcium silicate weight ratio greater than 16), obtaining a precipitate composed prevalently of CaCO3 in the presence of a solution containing nanometric particles of silica. In this system, however, the quantity of silica in the solution would be too low and therefore not sufficient for an industrial realisation of the process.
Even if the load of silica were increased, the concentration of silica in the solution would not exceed 10 kg/m3 (xc2x14); moreover in these experimental conditions this solution is a metastable phase which tends to gel quickly, preventing the separation of the silica from the precipitated solid composed of CaCO3.
It is known from the literature that the stabilising of a solution containing amorphous silica in particles is a function of the increase of thee pH up to alkaline valuesxe2x89xa79.5 or of the reduction of the concentrations of alkaline ions (R. K. Iler, The Chemistry of Silica, 1979). Alkaline pH values may be easily obtained by adding, for example, NaOH, while the problem of reaching higher concentrations of silica in the aqueous phase is more difficult to solve.
In order to solve this problem, the procedure according to this invention is characterised by the phase b), that is the reaction of the aqueous suspension 1, coming from the acid dissolution, with a compound of aluminium, boron or zinc or mixtures of the same in a neutral or basic environment.
The neutral or basic environment (pHxe2x89xa77) is preferably realised using a solution of hydroxides or salts of alkaline metals or alkaline earth metals. In particular, the solution is an aqueous solution of hydroxides of alkaline metals or alkaline earth metals and, even more particularly, the solution is an aqueous solution of sodium hydroxide.
The compound of aluminium, boron or zinc is preferably a salt chosen among aluminates, borates or zincates. In particular, the salt is an alkaline aluminate or an alkaline earth aluminate and, even more preferably, the salt is a sodium aluminate.
The treatment in phase b) is decisive for the development of the entire process, being essential for the separation, of silica from carbonate.
This reaction allows the modification of the silica surface and the increase of its concentration in the solution 3 at least up to 50-60 kg/m3.
The solution 3 which forms in phase b) contains particles of silica of with nanometric dimensions defined as xe2x80x9cprimary particlesxe2x80x9d. In particular the dimensions of the primary particles are between 1 and 100 nanometers.
The third phase c) of the process includes the separation of the solid phase 2 rich in calcium carbonate from the solution 3 containing silica.
The separation phase c) is carried out by means of any of the known methods for phase separations, preferably it is done by centrifugation; if the separated solid mixture still contains silica aggregated with carbonate, it may be recycled and again subjected to the treatment with aluminate.
The fourth phase d) of the process contemplates treatment of the solution 3 containing silica, according to one of the following methods:
according to the first method, indicated as e), the treatment may be carried out preferaby by drying or precipitation, obtaining precipitated silica.
Precipitation may come about by means of variation of the chemical-physical parameters, with or without the addition of precipitating agents.
In particular, precipitation may come about with the addition of CO2, at environment pressure and temperature. Afterwards there is the separation of the phases that formed during the precipitation phase. In particular, this separation may be achieved by means of filtration. The solid kept back by the filter is washed until a neutral pH (about 7) is reached in the washing waters. According to the method f), the treatment leads to gelation, followed by evaporation of the liquid phase. The gelation treatment, as known in the art, is achieved by acidification, for example by adding CO2, at environment temperature, to a pH lower than 7. Depending on the temperature at which evaporation takes place, cryogel, aerogel or xerogel may be obtained.
It is to be noted that the first phase of the process (reaction of calcium silicate with CO2) and the second phase of the process (treatment with salts containing aluminium, boron or zinc) may also be simultaneous and carried out in the same reactor, adequately checking the pH with suitable buffers.
A further aim of this invention is the use of the amorphous silica obtained with the procedure according to this invention as an additive in mixtures for tyres with low rolling resistance force or as a mineral addition in High Performance Concretes (HPC), such as high or very high strength concrete (preferably more than 100 MPa).
The term High Performance Concretes (HPC) identifies a class of cement mixes with characteristics of high mechanical strength (compression strength Rc at 28 days greater than 60 MPa).
An example of HPC is a high-strength concrete known as DSP (densified with, small particles). In this material the microstructure of the cement matrix is densified by adding silica fume. In DSP it is possible to reach Rc at 28 days greater than 100 MPa.
In particular, also the compound material composed of the mixture of silica and calcium carbonates, obtained according to the procedure of this invention, may be used directly as an additive in mixtures for tyres or in high-performance concretes, for example in a high-strength concrete (DSP).
Moreover the amorphous silica obtained with the procedure according to this invention may be used in many other sectors; it may be used as a reinforcing load in rubber and other organic polymers, as a pigment partially substituting TiO2 in the production of paper and coating, as a dimmer in the water paint and varnishes industry, as a thinner in solid formulations, as an anti-blocking agent to prevent adhesion between smooth surfaces, as an anti-binding agent or a catalytic support.
The principal advantage of the procedure according to this invention is that of obtaining high-purity amorphous silica by means of a process with a low environmental impact, presenting high production rate and high yield at the same time.
A further advantage of this invention is that of obtaining precipitated calcium carbonate in the form of fine particles.
The starting material may be natural, synthetic, crystalline and amorphous calcium silicates.
The starting material may also be clinker of Portland cement. By clinker of Portland cement is meant a hydraulic material which must be composed of at least two thirds in mass of calcium silicates (3CaO.SiO2) and (2CaO/SiO2). The remaining part contains aluminium oxide (Al2O3), ferric oxide (Fe2O3) and other oxides. The CaO/SiO2 mass ratio must not be less than 2.0. The magnesium oxide (MgO) content must not exceed 5.0% of the mass.
Clinker of Portland cement is obtained by the baking, at least as far as sintering, of a precisely established mixture of raw materials (raw meal, paste or suspension) containing CaO, SiO2, Al2O3 and small quantities of other materials. The raw meal, paste or suspension must be finely ground, closely mixed and therefore be homogeneous. The clinker of Portland cement used as the starting material for the process of this invention is preferably rich in silicates and contains low percentages of the other oxides.
A calcium silicates which can be used as the starting material in the process according to this invention is xonotlite (Ca6Si6O17(OH)2) which may be synthesised in hydrothermal conditions according to the following reaction: CaO/Ca(OH)2+silica (or sand or quartz)+H2O+minor components (silica fume, bentonite) are made to react in the presence of water vapour at a temperature between 176 and 240xc2x0 C., at a pressure of 1.4-1.5 MPa, for a time of 1 to 24 hours, preferably from 1 to 7 hours, with a molar ratio CaO/ SiO2 between 0.65 and 1 and with a specific surface of the reagents from 18 to 22 m2. After cooling, filtration and washing, crystals of xonotlite are obtained.
It is also possible to use as starting material a calcium silicate which may be crystalline or prevalently amorphous. In particular, feeding pure quartz with a diameter less than 40 xcexcm, CaO (containing about 7% CaCO3, with diameter  less than 25 xcexcm), silica fume and bentonite in a one-litre stainless steel autoclave, lined, with agitator, and letting it react at a temperature between 195 and 200xc2x0 C. for 24 hours at a pressure of 1.53 MPa, prevalently crystalline calcium silicate is obtained. Instead, using reaction times of 4-7 hours, prevalently amorphous silicates are obtained with a surface area equal to approximately 100 m2/g.
Another calcium silicate that may be used as a starting material is tobermorite.
The following tables 1 and 2 list some conditions of synthesis of xonotlite (xon-1, xon-2, xon-3) and of amorphous hydrate calcium silicates (CSH-1, CSH-2, CSH-3, CSH-4 and CSH-5) which may be used as starting materials for the procedure according to this invention.